1. Field of the Invention
The present invention relates generally to signal generating methods and devices and, more particularly, to a phase persistent agile signal source method, apparatus, and/or computer program product.
2. Description of the Related Art
Signal frequency generation can be achieved in a number of ways including direct digital frequency synthesis, phase-locked-loop frequency synthesis, fractional-N frequency synthesis, etc. Advances in integrated circuit technology over the recent past have resulted in more widespread use of direct digital synthesizers (DDSs) in signal frequency generation. DDSs generate programmable analog output waveforms with high resolution and accuracy, and are able to rapidly switch between output frequencies. A DDS generates a waveform by storing the points of the waveform in digital format, and then recalling them to generate the waveform. The rate at which the DDS completes one waveform governs the output frequency.
The DDS output frequency is changed by changing the phase increment of the phase accumulator. The phase increment determines how many data points the DDS skips between the ones it sends to the digital-to-analog converter (DAC). The DDS accumulates the skip values to determine the values that are sent to the DAC. This accumulator has history of all prior programmed frequencies. When the DDS output frequency changes from a first frequency to a second substantially higher or lower frequency, and then back to the first frequency, the second occurrence of the first frequency normally does not have the same phase as the first occurrence of the first frequency. This is due to the history of all prior frequencies stored in the accumulator of the DDS. It would be desirable to track phase of the output frequency of a DDS to maintain the time continuous phase of the DDS output frequency when the DDS is programmed to any other arbitrary output frequency. If the DDS output frequency is reprogrammed to a previous DDS output frequency, the phase would then be continuous, as though it never left the original frequency.
Therefore, a need exists for a phase persistent agile signal source method, apparatus, and/or computer program product.